Optical disc unit and method and apparatus for writing and/or reading information thereon

ABSTRACT

An improved method of optically writing and/or reading high density information involves: (1) predeterminedly positioning a flat, uniformly thick, flexible, optical disc with respect to an axis of rotation; (2) rotating the disc on such axis at a high speed and in a predeterminedly spaced relation to a predeterminedly shaped, smooth, featureless reference surface and (3) coupling a central region between the disc and reference surface to an ambient gas source. Related apparatus and disc configurations, including embodiments having flexible disc covers, also are disclosed.

This is a division of application Ser. No. 160,769, filed June 18, 1980,now U.S. Pat. No. 4,447,899.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the optical recording and/or playbackof high density information and more particularly to improved methods,media and apparatus for implementing such recording and/or playback in aflexible disc format.

2. Background of the Invention

The currently preferred optical disc technology employs disc elementswith spiral or concentric tracks of minute (usually on the order of amicron or less in size), optically-detectable marks. One real-time modeof recording (writing) such marks is by scanning tracks on the disc withan equivalently small beam of radiation (e.g., from a laser) which ismodulated "off or on" according to an electrical signal representativeof the information to be written. Information is recovered (read) byscanning the tracks with a larger, but still very tightly focused lightspot. The recovered information is in the form of a fluctuatingelectrical signal obtained from a photodetector that senses the read-outlight reflected from the recorded disc.

In order to write and read information in the form of such minutemarkings, optical systems of high numerical aperture are used to focuslight to equivalently minute spots. Such optical systems have extremelysmall depths of focus and the proper positional relation between thewriting or reading optical system and the optical disc surface must bestringently maintained. One approach has been to utilize highly-preciseturntables for supporting and rotating the optical discs in apredetermined plane. Such apparatus is expensive, but even with suchcostly turntables it is usually necessary also to provide complexfocus-servo devices which effect lens adjustment in response to minutevariations in the position of the surface of the optical disc relativeto the lens. Such variations are caused by thickness variations andnon-flatness of the disc or turntable surface or apparatus vibrations.Since the discs rotate at high speeds (e.g., 1800 RPM), thesefocus-servo devices must respond at very high frequencies (e.g., 500 Hz.for discs with ground glass substrates and in the range of 1000 to 3000Hz. for discs with molded plastic substrates). Therefore thesefocus-servo devices are also costly, and fragile.

The costs and care of such sophisticated writing and reading apparatushave not presented insurmountable problems for some industrial orlaboratory applications, but do present significant obstacles forconsumer and business systems applications. Therefore, an alternativehas been suggested whereby master discs, formed with the sophisticatedapparatus described above, will be used to form replicate discs onflexible substrates, such replicate discs being intended for playbackonly. It is suggested that such flexible discs be air-supported duringrotation in a manner similar to that previously used for magnetic discs.That is, it had been known previously that a "floppy" disc, comprising amagnetic layer on a readily-flexible, plastic support, could be rotatedin a generally stable condition by providing an opposing stationaryreference surface and supplying a throttled air flow between the discand reference surface from a location near the rotational center to thedisc periphery (see, e.g., U.S. Pat. No. 3,208,056).

The approach described in the above noted patent relied on the rotatingdisc achieving a balance of dynamic, fluid, gravitational and elasticforces so as to generally stabilize in some equilibrium range whenrotated at a fixed speed. In accordance with the teachings of thatpatent, fluid forces were regulated (by controlling the air flow betweenthe disc and stabilizing reference surface) to selectively position thedisc relative to magnetic transducers embedded in the stabilizingreference surface. This allowed compensation for the differences indifferent discs' equilibrium positions or changes in ambient fluidconditions. It is significant to note that the aim for this magneticwrite-read application was to provide a disc to transducer spacing of0.001" or less and that the stabilizing reference surface includedcavities as well as the embedded transducer heads. In a subsequentpatent (U.S. Pat. No. 3,191,179) of the same assignee it was pointed outthat there still existed instability problems in devices of the kinddisclosed in U.S. Pat. No. 3,208,056. It was suggested thatelectrostatic forces be used to remedy such problems.

Subsequent development of this aerodynamic stabilization approach (seeU.S. Pat. No. 3,178,719 of the same assignee) led to the implementationof what was termed a "regulated mode" of operation. In accordance withthe regulated mode, the rotating disc was generally stabilized asdescribed in U.S. Pat. No. 3,208,056 and then locally regulated orconstrained toward the magnetic heads, e.g., by a localized vacuumproximate the head.

In prior art attempts to adapt the magnetic disc aerodynamicstabilization for the reading of optical discs, the regulated modeapproach has been pursued. More specifically, although there has beendisclosed various embodiments of optical disc reading apparatus whichutilize a stationary reference, a rotating flexible disc and an air flowtherebetween, all such embodiments additionally utilize some localizedregulating means at the optical detection site to constrain the discinto a precise focused position relative to the optical system. Typicalexamples can be seen in French Pat. No. 2,167,258 (wherein adihedral-shaped reference surface and localized stabilization plateseffect such constraint) and in U.S. Pat. No. 3,947,888 (wherein anegative pressure differential in the vicinity of the reading headconstrains the disc into a predetermined head-to-disc spacing). Aninteresting commentary on the problems which have existed is in "TheAerodynamic Stabilization of Video Discs", IEEE Transactions on ConsumerElectronics, Vol. CL-21, No. 2, May 1975, wherein another "regulatedmode" embodiment combining aerodynamic prestabilization and localizedconstraint is disclosed.

Thus, prior art techniques for rotating flexible optical discs on an aircushion have followed the regulated mode approach with various localizeddisc-constraining devices. Such devices add to the cost and complexityof the apparatus. Moreover, even with such constraining devices, priorart techniques have only been suggested for use in reading (not writing)with flexible optical discs. The focus precision requirements forwriting on optical discs are even more stringent than for reading suchdiscs. For example, typical optical disc systems for reading would havedepth of focus tolerances of from ±3μ to ±7μ (depending on theinformation storage density of the disc), whereas the correspondingsystems for writing information on such discs would have tolerances offrom ±0.25μ to ±1μ.

SUMMARY OF THE INVENTION

The present invention provides an alternative approach, with respect tothe "regulated mode", for using flexible optical discs. Morespecifically it is one advantage of the present invention to facilitateuse of flexible optical discs, in an aerodynamically stabilizedcondition, without any localized forces or structure which constrain thedisc out of an equilibrium path and into some predetermined positionrelative to the optical system. It is a further advantage of the presentinvention to stabilize flexible optical discs in a manner much moreprecise than envisioned by the prior art, e.g. so that such discs can beeither written or read, rather than only read. Related advantages of thepresent invention are to provide improved apparatus and discconfigurations which facilitate such an improved mode of operation.

In another aspect, it is an advantage of the present invention toprovide improved flexible cover sheet constructions for optical discs.

In general, these objectives are accomplished in accordance with thepresent invention by (1) rotating a flexible, substantially-flat opticaldisc, having a substantially-uniform thickness, about an axis normal tothe disc's central surface with the disc in opposed relation to asubstantially-smooth stabilizing surface which is axially symmetricabout the axis and generally non-convex with respect to the plane of thedisc's central surface and (2) coupling a radially central region of thezone between the disc and reference surface to a gas source of pressurenot substantially exceeding ambient pressure. It is preferred that therotational speed of the disc and the spacing of the reference surfacefrom the disc be such that no portion of the reference surface is closerthan 0.001" to the disc and that the distance between the disc peripheryand the opposing reference surface portion does not allow excessiveturbulence at disc periphery. While rotating in such a stabilizedcondition, a tightly focused light beam is modulated to writeinformation along tracks of the disc or scanned along recorded tracks toread recorded information on the disc. It is highly preferred inaccordance with the present invention, that the optical system notconstrain, or otherwise interfere to any substantial extent, with theequilibrium path of the rotating disc. In one preferred mode, theoptical system is adjusted by a "gross" control system in accordancewith the radial profile of the stabilized disc.

In accordance with further aspects of the present invention, improvedapparatus and flexible optical disc configurations and combinations areprovided for implementing the above-described methods. The apparatus ischaracterized by means for supporting and rotating such a flexibleoptical disc in the manner described above, reference surface means forcooperating with such rotating disc in the manner described above, meansfor coupling a radially central region between a supported disc and thereference surface means to a gas source of pressure not substantiallyexceeding ambient pressure and means for optically writing and/orreading information upon such a supported, rotating disc. The improvedflexible, optical disc is characterized as being substantially-flat, ofsubstantially uniform thickness and as having a substantiallycylindrical outer periphery.

In accordance with another aspect of the present invention, such aflexible optical disc further includes a flexible protective disc coverhaving a shape corresponding to the disc and having a central regionthat is secured in a fixed, coplanar and predeterminedly spaced relationwith respect to a corresponding central region to the optical disc, suchdisc cover being substantially optically transparent to writing and/orreading wavelengths and non-birefrigent and having means defining aradially central air access opening to the space between the opticaldisc. Thus, during rotation of the optical disc such disc cover willmove to a generally stabilized position out of the depth of focus ofoptical means focused (to write or read) upon the optical disc. Duringstorage the cover will provide a protective surface to prevent theaccumulation of dust and scratching on the surface of the optical disc.It is highly preferred that such disc cover be flat, opticallyhomogeneous and of uniform thickness. In another aspect of the presentinvention, such a protective disc cover includes filter means overlyingsuch central air access opening.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe understood further from the subsequent description of preferredembodiments which is made with reference to the attached drawingswherein:

FIG. 1 is a schematic view of one embodiment of optical disc write/readapparatus with which the present invention is useful;

FIG. 2 is a perspective view illustrating one embodiment of apparatusand mode of the present invention;

FIG. 3 is a cross-sectional view along the line III--III of FIG. 2;

FIGS. 4 and 5 are respectively enlarged side and top views showing inmore detail the relation of portions shown in FIG. 3;

FIG. 6 is a cross-sectional view of one preferred embodiment of opticaldisc in accordance with the present invention;

FIG. 7 is an enlarged diagrammatic side view showing the operativerelation of portions of the optical disc and reference surfaceillustrated in FIG. 3;

FIG. 8 is a cross-sectional side view, with portions broken away, of oneembodiment of flexible optical disc and flexible disc cover according tothe present invention; and

FIG. 9 is a diagrammatic side view like FIG. 7, but showing theoperative relation of the FIG. 8 element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates one preferred write/read apparatuswherein the present invention can be utilized with extremely goodresults. The writing means comprises a writing laser 1 which directslight of a first wavelength through a polarization rotator 2 toacousto-optic modulator 3 adapted to receive an electrical informationsignal and modulate the writing light beam in response thereto. Lightexiting modulator 3 passes through afocal telescope optics 4 and isdirected via mirrors 5 and 7 and beam-splitter 6 to a negativecorrection lens 8 and the focusing objective 9, e.g., a biologicalobjective such as a Nikon Achromat NA 0.65 biological objective. By thisoptical means, light from the laser 1 can be controlled to form tracksof micron-sized pits in the rotating disc D as it moves above thestabilizing reference surface R. Preferred disc and reference surfaceconfigurations will be explained in much more detail subsequently.

The reading means shown in FIG. 1 comprises a reading laser 10 whichdirects light of a second wavelength sequentially through polarizingrotator 11, polarizer 12, tracking modulator 14, afocal telescope optics15, polarizing beamsplitter 16, one-quarter wave plate 17, beamsplitter6 and mirror 7 to elements 8 and 9. Light from the focused read spot isreflected from the disc D and returns through elements 9-6 and 17 topolarizing beamsplitter 16. The reflected read light is then directed toinconel beamsplitter 20 where it is divided and directed both to asignal detection system 21 and a focus detection system 23. The focusdetection system 23 receives read light via bandpass filter 24 and afocusing lens 25. System 25 can be one of various kinds known in theart, e.g., including a Foucault knife and a split diode. The readdetector system 21 similarly can take various forms known in the art.One example of such a system is disclosed in U.S. Pat. No. 4,065,786.

Referring now to FIG. 2, one preferred embodiment for effecting precisestabilization of a flexible optical disc is shown. Within the write/readapparatus shown in FIG. 1, a rotatable drive shaft 31 is mounted inhousing 30 with its rotational axis normal to the deck 33. Meansdefining a stabilizing reference surface 34 is adjustably supported onthe deck 33 in concentric relation around shaft 31. The optical disc 13is clamped by cap 36 to the top 35 of the drive shaft 31, which islocated at a predetermined height above the reference surface 34.

More detail of the interrelation of the FIG. 2 elements is shown inFIGS. 3-5 where it can be seen that shaft 31 is coupled to motor 37which is adjustably supported by spider elements 38 to facilitateadjustment of the shaft axis to a condition generally normal to the deck33, with its top surface 35 at a predetermined height above stabilizingreference surface 34. The top surface 35 is precisely machined to assureit is normal to the shaft axis of rotation. In accordance with a highlypreferred mode, the disc-aligning portion of this surface has avariation of less than ±0.2 milliradians from the plane normal to theaxis of rotation, when measured by rotating the shaft by hand.Adjustable elements 39 are provided to assure reference surface 34 isprecisely parallel to top surface 35 and precisely normal to the axis ofrotation of shaft 31.

As shown best in FIG. 4, clamp 36 has a protruding annular clampingportion and a centering portion which is adapted to fit snugly in arecess in the top of shaft 31. A precisely centered and sized aperturein disc 13 thus can cooperate with the centering portion of clamp 36 soas to precisely radially align the disc center with the axis of shaftrotation. When clamp 36 is in place in the shaft recess and secured,e.g. by a bolt 40, the annular clamping portions around its peripherysecure the underlying portions of the disc in precise parallel relationto the shaft top 35 and thus the top of reference surface 34.Additionally, this peripheral clamping isolates the exterior peripheralportions of the disc 13 from any stresses created by the formation ofits central aperture. It is also highly preferred that peripheries ofclamp 36 and shaft 31 have a variation not exceeding 0.0001" withrespect to their radial distance from this axis of rotation. This isimportant so that the engagement zone with the disc is axiallysymmetrical and will initiate uniform stresses within the disc. It alsois highly preferred that the shaft have at least a predetermined minimumdiameter, in relation to a given diameter disc, so that the disc is notexcessively stressed, either in the radial or tangential directionsduring rotation. For a 12" disc rotating at 1800 RPM, we have found ashaft diameter of 11/8" to be highly useful.

It will be roted that the central aperture in reference surface 34exceeds the diameter of shaft 31 so as to leave an annular air passagefrom the atmosphere to the radially central region of the zone areabetween disc 13 and reference surface 34. In accord with the preferredform and mode of the present invention, this annular passage is betweenthe 11/8" outer diameter shaft 31 and inner diameter opening of 11/4" inreference surface 34. However, the size of this opening can vary so longas air is not so severely throttled as to cause contact between the discand reference surface, during rotation, and the air flow into the gap isquiet (i.e., not turbulent). As shown in a preferred embodiment, thepassage communicates freely with ambient atmosphere via the spacebeneath the reference surface and the interior of housing 30.

Before proceeding to a description of how the apparatus shown in FIGS.2-5 rotates disc 13 for writing and reading such as by the apparatusshown in FIG. 1, it is appropriate to describe features of the disc 13itself in more detail for certain of its features are important foroptimal practice of the present invention. FIG. 6 illustrates a portionof one useful disc 13 in cross-section and it can be seen to comprise arecording layer 41, a reflective layer 42 and a support 43. Onepreferred construction for the reflective and recording layers isdisclosed in detail in U.S. application Ser. No. 124,381, entitled"Physically Optimized Optical Disc Structure, Method and Apparatus" andfiled Feb. 25, 1980 in the names of Howe and Wrobel. In general, therecording layer is a real-time recordable dye-binder layer ofpredetermined thickness and optical constants relative to the write andread wavelengths. The reflective layer could be, e.g., a 500 Å vapordeposited aluminum layer. There are of course a wide variety of otherrecording layers which could be utilized in discs constructed for thepresent invention and in some known disc configurations the reflectivelayer is not required. Thus the characteristics of layers 41 and 42 arenot critical in practicing the invention except insofar as they could beconstructed to adversely affect the important characteristics regardingsupport 43 that are outlined below.

Specifically, it is important that optical discs for practice of thepresent invention have good flatness and thickness characteristics. Itis also important and highly preferred that the disc have a smooth,substantially cylindrical periphery which is radially symmetric to itscentering aperture. Moreover, we have found that in order to achieve ahigh degree of aerodynamic stabilization throughout a large annularportion of the disc, which is desired for optimal practice of thepresent invention, the flexible disc 13 preferredly should be"substantially-flat" and of "substantially-uniform-thickness." Not allcommercially available support materials meet the "substantially-flat"and "substantially-uniform-thickness" criteria, therefore general testsare described for evaluating support materials according to eachcriterion and a preferred example of such a support material isidentified below.

Herein the term "substantially-flat" as used in reference to thesupport, and disc itself, is intended to define the characteristic ofhaving extremely little curl and core set. As a test for a preferredsupport material, a thin, e.g. 4 mil, 12" diameter disc element formedof the proposed material should be placed on a planar surface in anunconstrained condition. If such disc element exhibits any portionperipheral or central, that raises more than about 1/32" from the planarsurface, it does not have the optimal characteristic of being"substantially-flat". It is highly preferred that such element have noportion that raises more than about 1/64" in such test. One preferredmaterial which has been found to have such substantially-flatcharacteristic is an Estar polyethylene terephthalate film base which isextruded and biaxially oriented and commercially marketed as thephotoconductor support of the Kodak Ektaprint L Image Loop. It is to beunderstood, however, that beneficial results can be obtained inaccordance with the more general aspects of the present invention withmore generally flat support materials, not meeting the above, preferredcriterion. For example, good stabilization of discs over useful annularportions thereof have been attained with disc supports having peripheralcurl of 1/4" or even 1/2".

Herein, the term "substantially-uniform-thickness" as used in referenceto the disc support, and the disc itself, is intended to define thecharacteristic of having total thickness variation of about ±2.5μ orless. The Estar film support identified above also meets this definedcharacteristic. Again, it is to be understood that support materials ofmore generally uniform thickness may be useful in certain applicationsin accordance with the present invention. For example, where depth offocus is less critical such as in read-only applications, usefulmaterials could have thickness variations in excess of the definedcriterion.

Having described the useful and preferred disc characteristics, thepreferred structure and mode for rotating the disc in a highlystabilized condition will be explained. In general, it is extremelyimportant, for optimal practice of the invention, that the stabilizingreference surface 34 be properly configured and that the height of thetop surface 35 of shaft 31 (and thus the disc's orientation plane) beproperly positioned relative to reference surface 34.

More specifically, the preferred reference surface configuration hasthree important characteristics. First, it should be aerodynamicallysmooth and featureless. That characteristic of "featureless" denotes acontinuous surface in distinction to prior art techniques in whichgrooves, apertures or sensors were incorporated in the surface. Turnedand polished metal surfaces or chrome plated surfaces have been found tobe preferred from the smoothness viewpoint. Second, it is preferred thatthe reference surface be radially symmetric with respect to the axis ofrotation of shaft 31 and non-convex with respect to the plane of theshaft top 35 (i.e., the plane of disc orientation). The planar surfaceprovided by the annular reference member 34 in the illustratedembodiment of course meets this requirement. However, it may bepreferred to modify the surface configuration of reference member 34 tomake it symmetrically concave, e.g., as a surface or revolution definedby a curve sloping upwardly from the central axis to the periphery. Suchmodifications can be useful to cause the stabilized condition of therotating disc to be more planar in nature. In regard to symmetry, wehave found it to be highly preferred that, at given radii, the referencesurface have less than 1 mil (most preferably ±0.5 mil) variation inheight from a plane normal to the axis of rotation of shaft 31. Finally,the periphery of the reference surface should extend at least to theperiphery of the disc. That is, it should not terminate substantiallyradially inwardly from the disc periphery or air turbulence will occur,disrupting any effective stabilization of the rotating disc.

As mentioned above, the height of the reference surface 34 in relationto the top surface 35 of shaft 31 is also important. The preferredlimits for the variation of this distance ("h" in FIG. 7) can best bedescribed functionally in regard to the certain peripheral conditionscreated between the disc and reference surface during disc rotation.That is, the maximum preferred distance for height h is one which; whenthe disc is rotating at its nominal speed, just avoids the condition ofexcessive turbulence at the disc periphery. For a disc having a 0.004"thickness and a 12" diameter, rotating at 1800 RPM over a planarreference surface, such as shown in the drawings, this maximum height"h" occurs at about 0.015".

We have found the minimum for the height h to be a function of particleswhich are present in air and that it should in no event be less than asto maintain a peripheral air gap of about 0.001" between the disc andreference surface. Smaller peripheral gaps have two compounding adverseeffects which prevent any effective stabilization. First, dust particlesof 0.001" will become caught in the passage and disturb the laminarnature of the air flow. Second, even if particles are not "caught" to anextent blocking air passage, they forcefully impact the too closelyopposing surfaces causing a build up of electrostatic charges. Suchcharges have been recognized in the art as a significant deterrent tostabilized disc flight.

Another problem creating electrostatic disturbing forces can occurduring the start of disc rotation, i.e., its take-off. At this stagethere is inherently a frictional rubbing between the disc and thereference surface. The extent of this effect can be lessened to someextent by reference surface configuration (e.g., making it concave) andby choice of material (i.e., a chrome-coated reference surface exhibitsa good position in the triboelectric series relative to plasticsupports). However, we have found it to be extremely effective toprovide a quick disc take-off, e.g., preferredly effected by use of ahigh torque motor and relative light-weight construction of the movingparts. Alternatively peripheral lifter elements, e.g., solenoids 45shown in FIG. 3, might be used; however, these should be preciselyconstructed and quickly retracted after disc take-off, to a conditionleaving the reference surface featureless.

Referring now to FIG. 7, one preferred mode of operation according tothe present invention will be described. With the shaft 31 preciselyoriented so that its surface 35 is normal to its axis of rotation, adisc 13 such as described above is precisely centered and clamped into aflying orientation defined by the plane of surface 35. The stabilizingreference surface 34 such as described above, is precisely leveledrelative to the plane of surface 35 and located at a height h relativeto the plane of surface 35 so as to be well within the maximal andminimal conditions outlined above. The high-torque motor is actuated torotate shaft 31 and attached disc 13 quickly up to the desired operatingspeed, e.g., in less than 10 seconds. As the disc rotates over thestationary reference surface 34, dynamic forces tend to "fly" the disc,i.e., straighten it to a planar condition modified to some extent bygravitational forces and its internal elastic forces. Additionally, theair between the disc and reference surface is set into rotational motionby rotational movement of the disc surface. Due to centrifugal dynamicforces, the air flow spirals radially outwardly toward the peripheralgap between the disc and reference surface and air is drawn into thecentral region between the disc and reference surface through theannular space between the reference surface and shaft 31. This causes agradient pressure to develop radially outwardly.

The nominal disc rotational rate can be selected for the signal formatbut should be sufficiently high so that dynamic forces straighten thedisc to the modified planar condition above the reference surface. Theair supply to the central region is preferredly at atmospheric pressure,but we have found that good stabilization can be obtained with slightlynegative pressure sources. However, the pressure source should not besignificantly positive with respect to atmospheric. Also, we have foundit preferable that the air introduction means provide uniform flow alongits circumference, thus the annular opening between the shaft andreference surface is preferred.

We have found that if a properly configured disc is properly supportedfor rotation and if this outward movement of air is highly uniform inall radial directions, a highly stabilized equilibrium condition can beattained between the dynamic, elastic, fluid and gravitational forcesacting on the disc. Variations in symmetry of the radial fluid paths anddiscontinuities along the paths disturb the disc portion passingthereover from any stabilized condition and may even set up vibrationalconditions which compound instability. Variations from the preferreddisc construction and support means can cause similar disturbances.However, by operating in the completely passive mode which has beendescribed and precisely selecting materials and configurations toconform with this mode, we have attained remarkable stabilization invertical displacement of a large annular portion of the disc surface.Thus, using recording layers of the type disclosed in U.S. applicationSer. No. 124,381, we have been able to write and read back pits having adimension of 0.6μ (cross-track width)×1.0μ (in-track length), which werespaced center-to-center by a distance of 2.0μ along the recorded track,at high signal levels without any active focusing servo mechanism.

In certain modes, it may be desirable to provide what we term a passivefocus servo system which can compensate for the very low frequencyvariation in disc height that occurs as writing and reading moves todifferent radial positions. Also such a focus servo can easilyaccommodate the minor variation in disc equilibrium position that existsbetween discs having similar, but slightly different characteristics(e.g. mass, internal elastic forces, etc.). Such a servo system isillustrated schematically as 23 and 50 in FIG. 1. It will of course beappreciated by one skilled in the art that an active focus servo canalso be utilized with benefit with the present invention so long as itis not of the type which constrains movement of disc. That is, it shouldbe moved toward and away from the disc (at a location not creating anysubstantial turbulence or localized pressure drop) rather than forcingor inducing movement of the disc surface to adjust focus.

The following example of one specific preferred embodiment and mode ofoperation will further illustrate the practice of the invention and itsremarkable results. Specifically then, a 300 mm diameter disc having asupport formed of 0.004 inch Estar film base of the kind describedabove, a surface smoothing layer, a 500 Å aluminum reflective overcoatand 1100 Å thickness dye-binder recording layer of the type described inU.S. application Ser. No. 124,381 was mounted in accordance with thepresent invention on apparatus such as shown and described with respectto FIGS. 1-5. The disc was rotated at 1800 RPM and optical measurementswere taken which indicated a stability of the recording surface of atleast ±3μ throughout a zone of from 60 mm radius to 115 mm radius. Therewere limitations as to the optical detection and it is thought thestability may be much better than ±3μ. Thus, as described above, therotating disc was written upon in several tracks, within a wellstabilized zone, by apparatus such as shown in FIG. 1, both with andwithout an active focus servo system. In another test, a 12-inchdiameter disc having a support of 0.004 Estar film base of the typedescribed above (with a 1/4" curl) was supported in apparatus aspreviously described with a height "h" of 0.010 inch and rotated at 1800RPM. Instead of a recording layer, the disc comprised a smoothing layeron the support and a 1000 Å aluminum layer. The rotating surface wasoptically measured, within a central annular portion, to be stable in adirection parallel to the axis of rotation to about ±1.5μ. Thus it willbe appreciated that this remarkable stabilization aspect of the presentinvention will allow significant improvement in the writing and readingof optical discs.

In accordance with another significant aspect of our invention, we havefound that when a flexible optical disc is stabilized in rotation asdescribed above, an attached protective, flexible disc cover rotatingtherewith can be aerodynamically stabilized with sufficient precision toallow high-density, high signal-to-noise ratio writing and readingtherethrough. This is highly desirable since high density optical discrecording surfaces are generally highly susceptible to damage byscratches, dust, skin oils or other similar contaminants. When arotating disc cover is elevated at a distance above the recordingsurface, the dust, debris and scratches on the surface of the cover areoutside the depth of focus of the lens focused on the recording layer.

Also, this mode and configuration, utilizing thin, transparentprotective layers (with an intervening air gap), considerably simplifiesthe correction of aberrations induced by focusing (with high ormoderately high numerical aperture objectives) compared to a continuousovercoat protective layer, which needs greater thickness to produce asimilar protective effect. For example, if disc cover thicknesses lessthan 0.18 mm are used, ordinary biological microscope objectives thathave working distance (the distance from the last lens element to thefocus) of 0.5 mm or more can be used. Thick, in situ, protective layerswould require special lens designs having very large working distanceand cover corrections and such lenses would necessarily sacrifice fieldof view and/or numerical aperture.

Referring to FIG. 8, one embodiment of this aspect of the presentinvention is illustrated. The optical disc 13 can be of the kindsdescribed above and preferably has the characteristics defined foroptimal stabilized rotation. Around the periphery of its centralaperture, the disc 13 has attached thereto a disc cover 83 which issubstantially equal diameter as disc 13. An annular spacer 84 couplesthe optical disc 13 and disc cover 83 and provides a predeterminedcentral spacing therebetween. The bonds 85 between elements 13, 83 and84 preferredly are adhesive, although other equivalent bonds could beused. Spaced radially outwardly a small distance from the spacer 84, thedisc cover has a plurality of apertures 86 arranged symmetrically aroundthe disc center and equidistant therefrom. Each aperture is covered by apermanent filter element 87 and a removable filter 88 for purposes whichwill be explained subsequently.

Certain parameters of the disc cover are important. It preferably isflat to less than about 1/2 inch over a 12" diameter when subjected tothe test described-above with respect to the optical disc. The aperturesshould be formed in the disc cover in a manner avoiding excessive strainor stress. It is preferred to have a thickness in the range from about 1mil to about 10 mils with a thickness variation in the order of ±2.5μ.The upper limit of the preferred thickness range and the thicknessvariation tolerance are related to the lens requirements necessary tooperate through the disc cover and the lower limit of the preferredthickness range relates to buckling during initial rotation of the unit.Additionally, preferred disc covers are substantially transparent to thewrite and read wavelength, substantially non-birefringent, substantiallyhomogeneous and free from striae and significant occlusions. Onepreferred material which provides these characteristics is thesolvent-cast, cellulose tri-acetate support of Eastman Color NegativeFilm 5247.

Referring to FIG. 9, the mode of operation of the FIG. 8 disc-disc coverunit 80 will be described. The unit 80 is mounted as described abovewith respect to FIGS. 2-7 with the optical disc precisely positionedrelative to the reference surface. Upon rotation, the optical disc 13 isstabilized as described by its dynamic and elastic forces and the fluidforces effected by air flow between disc 13 and the reference surface34. Disc cover 83 experiences similar dynamic and elastic forces, andthe air between disc 13 and cover sheet 83 spirals outwardly to create apressure gradient on its upper surface. Remarkably, the disc 13 is sowell stabilized with respect to reference surface 34, that it, in turn,provides a reference surface which stabilizes disc cover 83 within goodoperable limits. We have found that the neutral axis of a disc coverconstructed and mounted as described above can easily be stabilized towithin about ±6 mils.

The position of the disc cover between the lens and the recordingsurface does not affect the focused write or read spot except insofar asit allows scratches or debris on its surface to interfere with thatspot. The preferred minimum operative spacing between disc 13 and disccover 83 thus will depend on the numerical aperture of the lensutilized, the thickness of the disc cover and the desired degree ofprotection from dirt and scratches on the external cover surface. For acover sheet having a refractive index of n=1.5, the table set forthbelow illustrates useful minimum operative spacings "S" (i.e., betweenthe recording surface and the bottom of the disc cover in the utilizedannular portion of the disc) that will provide the same degree ofprotection from dust, scratches, etc., as does a 1 mm thick, in situ,transparent overcoat of index n=1.5 used with a 0.5 NA lens. The tablelists S for various NA lenses and cover sheet thicknesses of 100μ,133.4μ and 175μ.

    ______________________________________    NA       S (100μ)                       S (133.4μ) S (175μ)    ______________________________________    0.5      0.545 mm  0.5246 mm     0.4991 mm    0.6      0.4085 mm 0.3891 mm     0.3648 mm    0.65     0.353 mm  0.3342 mm     0.3108 mm    0.7      0.3032 mm 0.2853 mm     0.2629 mm    0.75     0.2578 mm 0.2408 mm     0.2196 mm    0.8      0.2152 mm 0.1994 mm     0.1797 mm    0.85     0.1743 mm 0.1601 mm     0.1423 mm    0.9      0.1332 mm 0.1211 mm     0.1059 mm    0.95     0.08814 mm                       0.0792 mm     0.06797 mm    ______________________________________

In view of the "S" values shown in the above table, it will beappreciated by one skilled in the art, normal working distances (thevalue of S+0.001" to 0.004" for clearance between the top of the coverand the lens) pertain, thus biological objectives can be used.

The nominal height of the cover sheet over the optical disc at itscentral region (i.e., the thickness of spacer 84) will be greater thanthe spacings "S" described above and will depend on disc rotationalspeed, elastic forces in the cover sheet, etc. We have found thatbetween 5 mils and 15 mils is a good generally operable range for thenominal disc cover height.

For example, a preferred minimum operative spacing (between therecording surface and the bottom of the disc cover in the utilizedannulus) for the 300 mm diameter, 0.004 Estar support, optical discdescribed in the specific example above, is 6 mils, when a 0.65numerical aperture lens and a 5.25 mils disc cover made of the acetatesupport described above are used. For rotational speeds of about 1800RPM, a spacer 84 of about 10 mils provides such an operative 6-milspacing. A disc-disc cover unit such as just described was rotated at1800 RPM and recorded and read back by apparatus such as described withrespect to FIGS. 1 and 3 (objective 0.65 N.A.). When so written andread, the disc-disc cover unit performed as well as the disc exampleabove, tested without the cover sheet. That is, a signal-to-noise ratioin excess of 50 db was obtained.

The filter elements 87 and 88 are for removing particles from the airpassing into the zone between the disc cover 83 and the recording layersurface of disc 13 and subsequently obscuring the recorded informationor contaminating the recording surface. We have found it preferred touse removable pre-filters 88, e.g., a Millipore pre-filter(manufacturer's identification number AP 2009000) in conjunction withpermanent filter 87, e.g., Millipore 0.2μ (manufacturer's identificationnumber FGLP 09025) or 0.5μ (manufacturer's identification number FHLP0925) filters, so that the pre-filter can be periodically replaced toallow good air flow. The pre-filters 88 preferably have a pull-awayadhesive bond to peripheral portions of permanent filters 87. Suchfilters can be used with good results and are expected to provide along-lived arrangement because the airflow therethrough is small, e.g.,on the order of 5 ml/min.

It will be noted that the configuration shown in FIGS. 8 and 9 providesanother advantageous aspect of the present invention. Specifically, byallowing air flow to ingress between the disc and disc cover throughfiltered openings in the disc cover (rather than in the disc 13), twoadvantages pertain. First, the surface of disc 13 is maintainedfeatureless, both on its side facing reference surface 34 and on itsside facing the disc cover 83. Second, upon removal and storage therecording surface of disc 13 remains protected from atmosphericparticles.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A method of optically writing and/or readinghigh-density information, said method comprising:(1) positioning anoptical disc unit, including (i) a flexible record disc which isgenerally flat, of generally uniform thickness, with an annular recordzone on one surface, and (ii) a flexible cover disc that has: (a) anannular portion, which is generally flat and generally uniform inthickness, and in spaced, opposing relation to said record zone, and (b)a central portion that is secured in a fixed, parallel relation withrespect to a corresponding central portion of said record disc, with theplanes of said central portions in planes substantially normal to anaxis of rotation and with the other surface of said record disc closelyadjacent a highly smooth, featureless stabilizing reference surface thatis axially symmetric to said axis of rotation and generally non-convexto said normal planes; (2) maintaining said reference surface stationaryand said central disc portions in said plane while rotating said discunit about said axis at a high speed which flies said other surface ofsaid record disc in closely spaced opposing relation to said stabilizingreference surface and flies said cover disc annular portion in spacedrelation to the annular record zone of said record disc; (3) coupling aradially central region of: (i) the space between said record disc andsaid stabilizing reference surface and (ii) the space between the recorddisc and the cover disc, to gas sources of pressure not substantiallyexceeding the ambient pressure external of said zone; and (4) directinga beam of light for writing and/or reading information on said discthrough said rotating cover disc and into precise focus on said rotatingrecord disc.
 2. A method of optically writing high-density information,said method comprising:(1) positioning an optical disc unit, including:(i) a record disc comprising an annular record zone on one surface of aflexible support which is substantially flat and of substantiallyuniform thickness and (ii) a transparent flexible, disc cover that has:(a) an annular portion which is generally flat, of generally uniformthickness and in spaced opposing relation to said record zone and (b) acentral portion that is secured in a fixed, parallel relation withrespect to a corresponding central portion of said record disc, with theplanes of said central portions in planes substantially normal to anaxis of rotation and with the other surface of said record disc closelyadjacent a highly smooth, featureless stabilizing reference surface thatis axially symmetric to said axis of rotation and generally non-convexto said normal plane; (2) maintaining said reference surface stationaryand said central portions in said planes while rotating said disc unitabout said axis at a high speed which flies said other surface of saidrecord disc in closely spaced opposing relation to said stabilizingreference surface and flies said cover disc annular portion in spacedrelation to the annular record zone of said record disc; (3) coupling aradially central region of: (i) the space between said record disc andsaid stabilizing reference surface and (ii) the space between saidrecord disc and the cover disc, to gas sources of pressure notsubstantially exceeding the ambient pressure external of said zone; and(4) directing a beam of radiation through said rotating cover disc andinto precise focus on said record layer and modulating said beam towrite information on said record zone.
 3. An improved optical disc unitadpated for the optical writing and/or reading of high-densityinformation, said unit comprising:(1) a flexible record disc, adaptedfor precise rotational stabilization, and comprising an annular recordzone formed on a flexible support which is substantially flat and ofsubstantially uniform thickness and has a smooth substantiallycylindrical peripheral surface; (2) a flexible cover disc that has: (a)an annular portion, in spaced, opposing relation to the record zone ofsaid record disc, which is optically transparent to the writing and/orreading light wavelengths, and (b) a central region that is secured,relative to a central region of said record disc, in a fixed, parallelrelation; and (3) means for coupling a radially interior region(s)between said record disc and said cover disc to air external of saidregion(s).
 4. An improved optical disc unit adpated for the opticalwriting and/or reading of high-density information, said unitcomprising:(1) a record disc comprising an annular record zone formed ona flexible disc support which is generally flat and of generally uniformthickness; and (2) a flexible cover disc that has an annular portionopposite said record zone and a central region that is secured to acentral region of said record disc in a fixed parallel andpredeterminedly spaced relation; (3) means for predeterminedly spacingsaid annular zone and portion during disc unit rotation; and (4) meansfor coupling a radially interior region(s) between said record disc andsaid cover disc to air external of said regions.
 5. Apparatus foroptically writing and/or reading high-density information, saidapparatus comprising:(a) an optical disc unit including (1) a flexiblerecord disc, adapted for precise rotational stabilization, andcomprising a record zone formed on a flexible support which is generallyflat and of generally uniform thickness; (2) a flexible cover disc whichhas: (i) an annular portion opposite said record zone, and (ii) acentral portion that is secured to a corresponding central portion ofsaid record disc in a fixed, parallel relation; (3) means forestablishing a spaced relation between said record zone and said annularcover portion; and (4) means for coupling a radially interior region(s)between said record disc and said cover disc to air external of saidregion(s); (b) rotatable means for positioning such optical disc unit incentered relation to a rotational axis and with said central portions ofsaid record disc and cover disc in a plane precisely normal to therotational axis; (c) stationary means defining a highly smooth,featureless, stabilizing reference surface that is axially symmetric tosaid rotational axis and non-convex to said normal plane; (d) means forrotating said rotatable means at a high speed which flies said recorddisc outer portion in closely spaced relation to said reference surfaceand flies the outer portion of said cover disc in spaced relation to theopposing portion of said record disc; (e) means for coupling a radiallyinterior region(s) between said stabilizing reference and said recorddisc to a gas source not substantially exceeding the ambient pressureexternal of said region(s); and (f) means for directing a beam(s) ofradiation for writing and/or reading through said cover disc and intoprecise focus on said record zone.